Generally speaking an adjustable telescopic prop consists of two concentric tubes; an outer tube and an inner tube. The inner tube can slide telescopically within the outer tube in order to adjust the overall length of the prop. The outer tube is normally provided with opposing longitudinal slots and the inner tube is normally provided with a corresponding series of pairs of opposing holes along its length. In use the inner and outer tubes are telescoped to approximately the right length and a pin is placed through the slots and holes that line up with the slot. Fine adjustment is then made to extend the prop to the correct length by means of a collar with a thread on its inner surface which engages with a thread on the outer surface of the outer tube. Turning the collar moves it longitudinally along the outer tube. The collar abuts the pin so that movement along the outer tube can urge the pin along in order to extend the inner tube and thereby lengthen the prop.
To undo the prop once it is no longer required the collar is unscrewed and the pin removed, the inner tube is then retracted into the outer tube.